1. Field of the Invention
The present invention is related to testing apparatus for non-destructive testing of hysteretic properties of magnetic sheets. More particularly, the present invention relates to a single apparatus adapted to measure flux (Mrt), remanent coercivity (Hrc), switching field distribution (S*) as well as the parameters usually measured from a hysteresis loop such as coercivity (Hc) and loop shape.
2. Description of the Prior Art
My U.S. Pat. No. 4,816,761 issued Mar. 28, 1989, for "Apparatus for Measuring the Hysteresis Loop of Hard Magnetic Films on Large Magnetic Recording Disks". This apparatus was adapted to measure the hysteretic properties in the presence of an applied field. However, this apparatus could not measure the vertical axis of the hysteresis loop in absolute electromagnetic units (emu), thus, was incapable of measuring the magnetic flux in the magnetic sheets being tested. Apparatus for sensing flux in magnetic sheet material is described in IEEE Transaction on Magnetics, Vol. MAG-21, No. 5, Sep. 1985 at pages 1584 to 1589 entitled "A Hall Sensing Magnetometer for Measuring Magnetization, Anisotropy, Rotational Loss and Time Effects", by P. J. Flanders. This article explains that it is possible to measure flux of a small sample which is placed in a magnetic field with a Hall effect sensor so that only the magnetization of the sample is detected and not the field being applied to the sample. Alternatively, two identical Hall effect sensing elements may be placed in the presence of the applied magnetic field with the sample under test and electrically connected in series opposition so as to cancel out the effect of the applied field. In summary, the above apparatus is limited to testing small samples of magnetic materials that could be placed in an applied magnetic field such as that generated in the gap between two current carrying coils or in the small gap of a ring magnet.
My U.S. Pat. No. 4,816,761 was capable of measuring in an non-destructive fashion, using the magneto-optic Kerr effect, to determine the coercivity and hysteresis loop shape of magnetic samples. The latter article by P. J. Flanders describes a method of measuring the flux of a small sample employing a Hall effect magnetometer. To use this prior art method on large magnetic recording disks or sheets, it is necessary to cut the magnetic material into coupons or chips small enough to fit into the gap of a conventional electromagnet or the gap between current carrying coils.
There are a known variety of non-destructive techniques for measuring the flux of magnetic samples, all of which depend on the use of applications of Faraday's Law, i.e. the time variation of the sample flux detected by a playback recording head. When using the inductive playback recording head method, the sample must be moving at high speed in very close proximity to the playback head as occurs in magnetic disk systems. The distance that such heads are spaced from the magnetic sample is approximately 5 micro inches or 0.125 microns. Further, the requirement for the critical speeds and spacing using such inductive tests in not conducive to using this method in a manufacturing environment.
It would be desirable to provide an economic, predictable and repeatable non-destructive flux measuring apparatus that does not employ relative motion inductive techniques. Further, it would be desirable to provide a novel non-destructive Hall effect magnetometer to measure flux in magnetic layers such as occurs in disks, tapes and magnetic sheets. It would be desirable that the novel apparatus and method would be adapted to measure the hysteretic properties of moving magnetic steel sheets such as transformer sheet steel.
3. Summary of the Invention
It is a principal object of the present invention to provide apparatus for non-destructive testing of magnetic sheet materials.
It is another principal object of the present invention to provide apparatus for testing thin film magnetic recording disks during continuous production operations.
It is another principal object of the present invention to rapidly and efficiently measure the flux, remanent coercivity and switching field distribution employing a single set of remanent flux measurements.
It is another principal object of the present invention to provide flux measuring apparatus in which the distance between the magnetic material under test and the sensor is relatively large.
It is another principal object of the present invention to provide an apparatus in which the distance between the magnetic material and the magnetic field generator is also relatively large.
It is a general object of the present invention to provide flux testing apparatus in which the velocity or speed of movement of the magnetic material relative to the sensor is not critical to the measurement of the hysteretic properties.
It is a general object of the present invention to provide apparatus for measuring the flux and other hysteretic properties of a magnetic material using one or more galvanomagnetic Hall effect sensors or magnetoresistors.
It is another general object of the present invention to provide an economic and reliable simplified apparatus for measuring hysteretic properties of stationary or moving sheet materials.
According to these and other objects of the present invention, there is provided a magnetic field generator for magnetizing a spot on a piece of magnetic material to be tested. The magnetic spot is preferably moved past a stationary Hall effect sensor which detects the magnetic field emanated from the magnetic sample. The process of magnetizing a spot on a sample and detecting the field emanating from the same spot is repeated at different magnetization levels to provide a set of automatic measurements that are recorded in a memory of a controller or a computer. An analysis of the recorded data permits the automatic computation of the remanent flux, remanent coercivity and switching field distribution as well as other hysteretic properties to be explained hereinafter.